This invention relates generally to high resolution imaging telescopes and, more particularly, to techniques for reducing the size and cost of such telescopes. An imaging telescope has a primary optical element, usually a mirror in large telescopes, and an optical train of other elements, all of which cooperate to produce an image of a distant object at a focal plane. Present day high resolution imaging telescopes are constrained in size because of the costs of fabricating high precision mirror surfaces. It has been observed that the cost of fabricating large optical mirrors goes up in proportion to the third power of the mirror diameter. Telescopes of over two to three meters in diameter are prohibitively expensive. Furthermore, the weight of a telescope primary mirror is also a limiting factor if the telescope is to be used in space, since the cost of placing any load into orbit around the earth is dependent on its weight and volume.
Apart from mechanical innovations in the form of lightweight honeycomb structures, prior activity directed to reducing the cost of large telescopes has been centered on the field of adaptive optics. An adaptive primary mirror can be constructed using piezoelectric actuators to change the shape of the mirror surface to compensate for aberrations introduced either in optical elements or in a medium through which light is received at the telescope. This approach is used to correct atmospheric aberrations and requires a wavefront sensor to sample the phase front of an incoming reference wave, and a relatively complex control system to apply distortions to the primary mirror, to remove the effect of the aberrations. Theoretically, the same technique could be used to dynamically measure distortions introduced by a mirror, and to correct any such distortions, but there are a number of problems with this approach.
First, the adaptive mirror, which is sometimes referred to as a "rubber mirror," is itself an expensive item, and the requirement for multiple actuators, a control system, and a wavefront sensor all contribute significantly to the weight and cost of the telescope. Also, conventional adaptive optics telescopes use reference waves that are as distant as objects to be imaged, so that the reference phase is flat. This places signal-to-noise constraints on the wavefront sensor, which receives only a limited number of photons from an object to be imaged.
It is also been suggested to use a phase conjugation process to correct for aberrations introduced in optical components or in the transmission medium. In this approach, an incident probe beam is reflected from or passed through an optical element, and becomes distorted by the inherent deformities of the optical element. If the incident beam is then phase conjugated and reflected from or passed through the distorting element again, the aberrations introduced in the first pass are effectively canceled or removed during the second pass. This approach is better suited for telescopes that produce a high-power beam than it is for imaging a relatively weak image beam from a distant object.
Ideally, what is needed is a technique for correcting structural aberrations, introduced in the primary of the telescope, farther "downstream" in the train of optical elements, closer to the focal plane at which the image is produced. Then the structural requirements could be relaxed and cheaper, lightweight materials could be used in the construction of the telescope primary.
It will be appreciated from the foregoing that there is still a significant need for improvement in the field of large imaging telescopes, especially those intended for use in space. The present invention fulfills this need.